1. Field of the Invention
The present invention relates to synthesis of materials using combustion in powder metallurgy and, more specifically, to reduction-type combustion synthesis (RCS) of pure high-surface-area silicon (Si) used in electronics, solar energy systems, high-energy density materials, etc.
2. Description of the Prior Art
With the development of advanced technologies such as electronics and solar energy, pure silicon becomes an important material for the industry, and the demand for this product is constantly growing. On the industrial scale, reducing silicon dioxide with carbon in high-temperature furnaces typically produces pure silicon (see Special Materials in Pyrotechnics: VI. “Silicon—An Old Fuel with New Perspectives,” Fischbachstr. 16, D-90552, Rothenbach and Pegnitz, Germany). Current investigations on improving this process are described by N. V. Nemchinova, et al, in “Physical and Chemical Simulation of Carbothermal Production of High-Purity Silicon,” Contemporary Problems of Science and Education, Irkutsk State Technical University, Siberian Academy of Sciences, Russia. However, this conventional method is energy consuming and requires a relatively long process (days). Furthermore, the final product normally requires additional purification. Moreover, in some processes the by-product of this technology is carbon dioxide, which is responsible for the undesired greenhouse effect.
The attractive “green” approach to silicon and silicon-nitride production for use in solar cells is suggested by Murray, J. P., Flamant, G., and Roos, C. J. in “Silicon and solar-grade silicon production by solar dissociation of Si3N4,”, Solar Energy, 80(10), (2006) 1349-1354. In order to shorten the long and energy-intensive high-temperature purification process, the authors proposed a two-step process to produce silicon from silica: first, a carbothermal reduction is carried out in the presence of nitrogen to yield silicon nitride, and then the nitride is dissociated in order to yield silicon. The last step could be combined with purification of the silicon if the solar-grade silicon is the desired end product. Experimental results indicated that silicon nitride is dissociated to yield silicon with no detectable nitride content.
Two other similar methods of silicon production, i.e., an aluminothermy and magnesium reduction, are also well known. Zhihao Bao, et al, in Nature 446, 172-175 (8 Mar. 2007) describes chemical reduction of three-dimensional silica microassemblies into microporous silicon replicas. The authors demonstrated a low-temperature (650° C.) magnesiothermic reduction process for converting three-dimensional nanostructured silica microassemblies into microporous nanocrystalline silicon replicas. The intricate nanostructured silica microshells (frustules) of diatoms (unicellular algae) were converted into co-continuous nanocrystalline mixtures of silicon and magnesia by reaction with magnesium gas. Selective magnesia dissolution then yielded an interconnected network of silicon nanocrystals that retained the starting three-dimensional frustule morphology. The silicon replicas possessed a high specific surface area (>500 m2 g−1), and contained a significant population of micropores (0.20 Å). This process enables syntheses of microporous nanocrystalline silicon microassemblies for use in sensors, electronics, and optical as well as other applications.
However, the aluminothermic reaction results in the formation of aluminum oxide, which is a hard-to-separate phase, and the process of silicon purification is abnormally difficult and expensive. The magnesium-reduction method is more attractive because, contrary to Al2O3, an undesirable phase, i.e., MgO, can be easily leached from the as-synthesized product. However, this method is still energy consuming and is difficult for production of submicron powders.
Known in the art is the synthesis of fine and pure silicon based on chemical deposition from the gaseous or liquid phase. For example, U.S. Pat. No. 4,751,067 issued in 1988 to H. Levin discloses a process for making silicon from halosilanes and halosilicons. The process is carried out in a reactor adapted for continuously producing molten, solar-grade purity elemental silicon by thermal reaction of a suitable precursor gas, such as silane (SiH4). The reactor includes an elongated reactor body having graphite or carbon walls that are heated to a temperature exceeding the melting temperature of silicon. The precursor gas enters the reactor body through an efficiently cooled inlet tube assembly and a relatively thin carbon or graphite septum. The septum, being in contact on one side with the cooled inlet and the heated interior of the reactor on the other side, provides a sharp temperature gradient for the precursor gas entering the reactor and renders the operation of the inlet tube assembly substantially free of clogging. The precursor gas flows in the reactor in a smooth and substantially axial manner. Liquid silicon formed in the initial stages of thermal reaction reacts with the graphite or carbon walls to provide a silicon carbide coating on the walls. The silicon-carbide-coated reactor is highly adapted for prolonged use for production of highly pure solar-grade silicon. Liquid silicon produced in the reactor may be used directly in Czochralski equipment or in other crystal-shaping equipment.
For the purpose of illustration, some methods, compositions, and apparatuses for manufacturing silicon powders known in the industry are described in the patent publications given below.
U.S. Pat. No. 6,007,869 issued in 1999 to F. Schreieder, et al, discloses a process for preparing silicon granules with chlorine contamination below 50 ppm by weight by deposition of elemental silicon on silicon particles in a fluidized-bed reactor. This reaction has a heating zone below a reaction zone. The silicon particles are fluidized in the heating zone by means of an inert silicon-free carrier gas to produce a fluidized bed and are heated by means of microwave energy. The silicon particles are reacted within the reaction zone where a reaction gas comprises a silicon source gas and the carrier gas. The average temperature of the reaction gas in the reaction zone, while the gas is perfusing the fluidized silicon particles, is maintained at less than 900° C. The average temperature of fluidized silicon particles in the reaction zone, while they are being perfused by the reaction gas, is maintained at greater than 900° C.
Russian Patent No. 2327639 issued in 2008 to Yu. Kolmogorov, et al, refers to obtaining highly pure silicon that can be used in the production of solar elements. Pure silicon dioxide is melted at a temperature of 1900° C., and a mixture of powders of pure silicon and silicon dioxide taken at a stoichiometric ratio is introduced into the melt. The resulting gaseous silicon monoxide is then reduced in a gaseous phase with pure methane at a temperature of 2300 to 2500° C., and elementary silicon is produced. The invention allows upgrade of silicon purity and reduces production cost.
International Patent Application Publication No. 2006/041271 published in 2006 (inventors: N. Bekturganov, et al) discloses a method for production of pure silicon by aluminothermic reduction of silicon dioxide in silicon-containing phosphorous slag for solar cell manufacture. The solar slag is loaded into an open graphite crucible and is then heated in an induction furnace at the eutectic melting temperature, whereupon it is mixed with aluminum. The obtained silicon separated from the slag emerges on the slag's surface and is loaded with the new portion of slag and aluminum. The process is repeated several times until full sedimentation of the reacted slag and its separation from the silicon, which appeared in the top part of the reactor. The obtained silicon is crushed and sorted. The process allows silicon powder total purity of 99.99.
Japanese Unexamined Patent Application Publication No. JP2008037735 published in 2008 (inventor: T. Shimamune) discloses a method for manufacturing high-purity silicon that is pure enough to be used in a solar battery. The silicon is obtained by reducing silicon tetrachloride with zinc gas. In the manufacturing method, the zinc gas or a zinc-containing gas essentially comprising zinc gas is constantly sent to a reactor to induce a zinc-gas atmosphere therein, silicon tetrachloride in a liquid state is introduced to the reactor, a zinc reduction reaction is induced to produce silicon, and only a reaction-product gas is discharged from the reactor at the reactor terminal so that the produced silicon is locally accumulated inside the reactor. The produced silicon is locally accumulated inside the reactor in a silicon-melt-retaining tank, which is kept at a temperature equal to or higher than the melting point of the silicon.
International Patent Publication No. WO2007116326 published in 2007 (Inventors: T. Kaufman, et al) discloses recovery of silica from aluminosilicate-containing material and the production of solar and/or electronic grade silicon therefrom. In particular, the process for manufacturing silicon comprises subjecting highly pure and particulate silica in amorphous form to an in-flight plasma carbothermic reduction in a plasma reactor consisting of a plasma torch and a vessel suitable for plasma-associated chemical reaction of silica and collection of liquid silicon. The highly pure and particulate silica is preferably produced using a process for manufacturing silica and/or alumina from an aluminosilicate-containing material comprising mixing the material with a metal chloride, preferably calcium chloride, being in the form of a solution or slurry, subjecting the wet mixture to a granulation step, burning the granules at a temperature of 900 to 1300° C., leaching the obtained heated mixture with hydrochloric acid to obtain a salt solution and insoluble silica, separating insoluble silica from the salt solution, and recovering dry silica. Alumina may be produced by crystallizing AlC13 from the salt solution and heating the AlC13*6H2O crystals to produce alumina.
In the Journal of Material Research, August 1995, pp. 2073 to 2084, N. Rao, et al, describe synthesis of nanophase silicon, carbon, and silicon carbide powders using a plasma expansion process. Nanophase powders of Si, C, and SiC with narrow size distributions are synthesized by dissociating reactants in DC arc plasma and quenching hot gases in a subsonic nozzle expansion. The plasma is characterized by calorimetric energy balances, and the powders by on-line aerosol measurement techniques and conventional materials analysis. The measured nozzle quench rate is approximately 5×106 K/s. The generated particles have numerical mean diameters of approximately 10 nm or less, with Si forming relatively dense and coalesced particles, while SiC forms highly aggregated particles. According to the authors, SiC particle formation is initiated by nucleation of small silicon particles.
U.S. Patent Application Publication No. 20080054106 published in 2008 (inventors: R. Zehavi, et al) discloses a method of jet milling silicon powder of high purity in which silicon pellets are fed into a jet mill producing a gas vortex in which the pellets are entrained and pulverized by collisions with each other or walls of the milling chamber. The average particle size can be controlled by varying gas feed pressure, flow rates for feed and mill gases, position of the vortex finder, size of silicon pellets, and feed rate of pellets into the mill. With this method, it was possible to achieve a narrow size distribution of 0.2 to 20 microns.
U.S. Patent Application Publication No. 20090092899 published in 2009 (inventor: J. Treger) discloses a method that includes combining fumed silicon oxide with a metal to form silicon having an average particle size of less than approximately 100 nm. Such silicon can be incorporated into an anode of a lithium ion cell.
However, none of the above-mentioned methods or devices allows for effective production of high-surface area (>100 m2/g) Si powders with small amounts of impurities (<0.1 wt. %) and with particles at the submicron size of (50 to 200 nm). Thus, a need exists for an improved method and apparatus for producing a submicron silicon powder.